Conventional installation systems for underwing high-bypass engines having mid-length fan air ducts represent the latter stages of an evolutionary process which initially began with the earliest turbojet engines which, compared to present-day engines, were substantially smaller, simpler, and lighter in weight. These early engines and their operational systems, however, exhibited a relatively high degree of unreliability. Consequently, the engines were required to be replaced relatively frequently, such as, for example, every five hundred (500) hours of flight time although some engines were allowed to remain on wing for 1000 hours or so. The installation systems for such engines therefore comprised various means or features whereby the engines themselves and thrust reversers were capable of being easily and quickly removed from the overall powerplant assembly. Such means or features generally related to the engine-strut and operational systems connections, and the accessibility thereto.
As powerplant and noise reduction technology evolved, however, and high-bypass mid-length duct engines were adopted, the aforenoted installation systems, accordingly modified yet nevertheless basically incorporated into the high-bypass midlength duct engine assemblies, have presented substantial operational, economic, and safety disadvantages and problems. Over the past twenty years, for example, the number of connections between the engine and the airplane strut-wing for the various operational systems for the engine and ducting has tripled. In addition, the attachment of a core-mounted engine has evolved to the point wherein as many as six structural connections are required between the engine case and the strut, at least one of these connections being of the multi-bolt type. During the same aforenoted period of time, the size of conventional turbojet engines has also dramatically increased. Engine cores, for example, currently measure approximately six feet in diameter while the engine fan cases currently measure approximately eight feet in diameter. As a result of these diametrical dimensions, the various operational systems and structural connections are often not readily accessible, and quite difficult to reach, by the field mechanics. Often, the mechanics must stand upon other structural components of the powerplant assembly in order to in fact gain accessibility to the engine-strut and operational systems connections.
It may readily be appreciated that because of higher compressor pressure ratios and turbine temperatures a great amount of heat is generated within the engine core. This heat is of course transmitted to the core case per se as well as to the core case environment defined interiorly of the engine core case and the fan duct environment defined exteriorly of the engine core case. Some advanced engines have added complex case cooling systems but nacelle temperatures have increased and will greatly increase if engine systems crack or fail. In view of the fact that the structural mountings or connections are partially secured to the engine core case, and as a result of a substantial number of the operational systems connections being disposed within the aforenoted two environments, great amounts of heat are transmitted to these various mountings and connections. The hot mounting and systems connections cannot therefore be readily removed or disconnected in the field until permitted to sufficiently cool. Alternatively, special tools must be utilized by the field mechanics, and insulation pads must be interposed between the mechanics and the hot propulsion system structures in order to protect such personnel from being burned. Consequently, in view of the fact that a substantial number of operational systems and mounting connections must be disconnected and re-connected when replacing a particular engine within a powerplant installation, and furthermore, in view of the additional fact that the operational systems and mounting connections can only be approached in a cautious manner in order to insure the safety of the field personnel, an engine-change or replacement operation has become quite time-consuming, tedious, and potentially dangerous.
Continuing further, another feature, originally derived from the earliest low-bypass ratio turbofan engines and accordingly modified during the aforenoted evolutionary process so as to be accommodated within the present-day conventional engine installation systems, comprises the strut-hinged fan duct cowl sections. These C or D ducts, as they are commonly known, were provided in order to permit the necessary accessibility to the engine-strut and engine operational systems connections. The D ducts were originally designed to be quickly openable and as large in their size as possible so as to expose the largest engine interior area as possible. As the present day engines evolved, and in order to accommodate the particular engine-strut mounting systems, the D ducts were hinged to the engine strut at the upper end of each duct while the lower end of each duct was latched to its mating duct beneath the engine core case. As the size of the engines have become progressively larger and heavier, the D ducts could no longer be manually handled by field mechanic personnel. Consequently, the engines were provided with suitable hydraulic-controlled devices for actuating the D ducts between their open and closed positions and for retaining the same in their open position.
It is apparent that a major disadvantage of the D duct system is that a substantial amount of excess weight, as embodied within the hydraulic opening devices, and the half-ducts themselves, is imparted to the overall engine system. Such devices also materially add to the manufacturing costs of the engine systems. Still further, in view of the fact that the D ducts are hingedly secured to the engine strut and tightly clamped about the engine core or fan case so as to provide the requisite sealing properties, the D ducts actually form a complete redundant engine mounting system, whereby the load paths are duplicated even though less rigid than primary mounts to the strut. As the strut bends under weight and air loads, the ducts are also caused to deflect. This has caused cracking within the fan duct acoustic linings. Replacement of the same obviously results in high maintenance costs. Some duct hinge systems have been designed to float or to have limits of deflection after which they then become rigid. These are very complex and expensive and manufacturing tolerances tend to be uncontrollable.
Another operational disadvantage of the conventional D duct systems resides in the fact that each duct presents four exposed internal corner structures to the fan air duct flow. As all corner structures disposed within an air flow produce wake, boundary layer, and turbulence losses, the propulsion units do not operate as efficiently as would otherwise be possible.
The D duct mounting system has also proven to be an operational hindrance and at times quite dangerous. The latching mechanisms, for example, which clamp the lower portions of the ducts together and about the engine core case have often proven difficult to align and latch. As the thrust reverser system is also encompassed within the aft end of the D duct system, misalignment of the ducts naturally affects the operations of the thrust reverser system. This misalignment is also rendered more pronounced as a result of the various bending and deflection loads impressed upon the engine strut, the engine core, and the D ducts in view of the aforenoted interrelated connection system defined between the engine core, the D ducts, and the engine strut.
Still further, as the duct structure can effectively retain the engine upon the strut structure in the aforenoted redundant manner, should the engine-strut mounting system fail, the engine could fall toward the ground when and if the D ducts are opened for engine inspection, maintenance, or replacement. In addition, should a D duct become detached during flight, as has in fact occurred, for example, due to high engine bending deflections and failures in the latching system, substantial damage to the aircraft can result. In the aforenoted instance, the detached D duct was thrust aft and upwardly under the wing and ruptured a fuel tank.
Another area of considerable concern in connection with conventional powerplants is the disposition of the various engine accessories, fuel lines, hydraulic lines, and the like within the powerplant units. With conventional powerplant systems, the fuel, electrical, bleed air, hydraulic, and starter air lines are disposed about the compressor and burner sections of the engine in a wrap-around fashion. Consequently, such zones are heavily congested thereby severely restricting accessibility to the engine, systems connections, and the accessories and equipment. Quite often one system overlaps another necessitating removal of one system, or part thereof, in order to maintain the one underneath.
The aforenoted disposition of the various fluid lines within the powerplant is also quite potentially dangerous in that the same poses real fire hazards to the powerplant and the aircraft. As has in fact already occurred, should the powerplant experience a turbine disc or compressor disc burst, the disc can sever the fuel lines disposed within the upper regions or quadrants of the powerplant assembly on its way to cutting through the firewalls shielding the strut. In one occurred instance, the resulting fire was of such intensity and time duration that the outboard wing section was burned through and departed the airplane. Fire hazards are also posed by the disposition per se of the fluid lines and equipment within the upper quadrants of the heated zone of the powerplant. Burner case cracks permit combustion chamber burnthroughs to occur thereby igniting a fuel line or severely damaging engine accessories and equipment. Fuel lines routed through the top strut and fire walls are vulnerable to these possible sources of ignition.
A last undesirable feature characteristic of conventional underwing high-bypass mid-length duct engines resides in the particular engine mounting strut employed within the system. This feature is concomitant with respect to the aforenoted disposition of the fluid lines and the engine accessory equipment because often the struts employed within the engine mounting systems were specifically structured in order to accommodate the disposition of the fluid lines and accessory equipment. For example, the strut structures were provided with through-bores for housing the various fluid lines operatively associated with the engine. The struts would also have specific, irregularly shaped configurations in order to clear particular auxiliary equipment disposed within the engine region. In a similar manner, the struts would be provided with offset or cut-out sections in order to accommodate the translatable thrust reverser structure. As noted hereinabove, the D ducts were hingedly secured to the engine struts, and consequently, the struts were provided with suitable hardware for hingedly mounting the D ducts thereon. Obviously, these specifically structured engine mounting struts are quite costly to fabricate and substantially more difficult to tune for flutter control in order to optimize the engine support functions of the strut under all ground or flight conditions. Also, the interdisposition of the fluid lines and equipment within the strut region only serves to further congest and hamper accessibility within this area of the powerplant. Still further, if the strut production tooling and manufacturing of parts has to be changed for stiffness to prevent flutter after flight tests, it can be very expensive if the strut and systems therein are complex as they are today.